|The Mini Traction Machine (MTM) – a ball that rests on a rotating disk – can be used to compare the friction properties of different fully formulated turbine oils under various test conditions Source: Shell|
New testing and modelling methodologies in tribology – the science of lubricants – are helping to achieve trouble-free service and to raise the efficiency of turbine lubrication.
Ronald Bakker, Shell Global Solutions; The Netherlands
Outstanding lubrication of turbines and associated equipment to ensure long, trouble-free operational lifetimes is vital to all forms of power generation. While many of the high performance lubricants already on the market use the latest technology to help increase performance and efficiency, opportunities for further improvements are now emerging from new scientific methodologies in tribology being developed in the laboratory.
The backdrop for much of this development work is an economic climate that combines historically high energy prices with rising environmental concern. Lubricants are a key component in the overall value chain and can have a crucial impact on efficiency, emissions and productivity.
The latest scientific techniques employ wear modelling and simulation tests, ‘smart’ laboratory screening and extensive performance prediction models – all of which are designed to simulate the performance of lubricants in the real world. Through testing, tribologists can build a more detailed understanding of the physical characteristics and demands placed on modern-day turbine oils, in turn shaping product development pathways.
These new methodologies are being used to identify ways to further improve performance, elevate efficiency levels and address specific wear or deposit issues – even for turbines operating under the most severe conditions – and are driving the next generation of lubricants.
Applications of these techniques include the use of a new traction measurement device to analyze the relative performance of oils under various conditions. Shell Lubricants’ technology department has also developed a test to assess an oil’s tendency to form deposits.
Efficiency and friction
Lubrication efficiency depends heavily on having an oil film of adequate thickness. But the ability of the system and lubricant to form and maintain this thickness is affected by many factors including friction, viscosity, load, sliding speed and temperature. Lubrication efficiency can be increased by optimizing factors such as lubricant viscosity, lubricant viscosity-pressure coefficient (the viscosity behaviour of the oil at high pressure), and surface active additives in the formulation.
Five to ten years ago, most mineral-based turbine oils were made using Group I base oils. Today, the majority of modern turbine oils are based on more hydro-processed Group II and Group III oils in order to meet higher performance requirements.
However, as base oils have differing characteristics, it is useful to be able to model their friction coefficient accurately. To do this, Shell tribologists have been using a traction measurement instrument called a Mini Traction Machine (MTM) – a ball on a rotating disk that can be used to compare the friction properties of different fully formulated turbine oils under various test conditions.
As one might expect, Shell studies indicate that at various load and speed levels, Group III base oil lubricants consistently generate less friction than their Group II counterparts; this confirms that using higher quality, more refined and advanced base oil composition, supported by the latest additive technology, can lead to lower friction coefficients to improve lubrication efficiency.
But, more significantly, not all Group III turbine oils give a similar friction reduction. In Figure 1, oils A and B are blended using the same additive chemistry, but different types of Group III base oils. Oil B gives lower friction at all loads and speeds tested, clearly indicating that significant differences in friction behaviour between addivated Group III base oils are possible.
Data gathered in tests such as this are being used in further modelling aimed at predicting, and in turn enhancing, the performance of new lubricants in turbine applications.
In recent years, turbine users have reported lube oil ‘varnishing’ in their systems. This varnish can appear as a thin film deposit – coloured orange, brown or black – on the interior of lubricant systems and represents a significant performance issue. Varnish formation in servo-valves can cause the valves to stick or seize, leading to unit alarms, trips or fail-to-starts. Another costly concern is the formation of varnish on thrust or journal bearings, resulting in increased wear rates and accelerated oil degradation. Other problems caused by varnish include reduction of cooler performance, increased bulk oil temperatures and prematurely plugged filters and strainers.
The many and varied causes of varnish formation are still the subject of active research – although they include extreme oxidative stresses from high operating temperatures and catalytic wear metals.
Many manufacturers’ specifications cover the deposit-forming tendency of their oils in the field, yet none of the industry oxidation tests provide a clear indication. These tests are designed to assess the suitability of a lubricant for use as a turbine oil and to give an indication of expected life – but not to accurately predict varnish formation in service.
Modified Wolf Strip testing equipment, designed to assess the deposit forming tendencies of oils under conditions comparable to those in the field Source: Shell
Shell Lubricants’ technology department has therefore developed a screening test that could be used to indicate an oil’s deposit-forming tendency – a modified Wolf Strip test (ex DIN 51392) that can evaluate an oil’s resistance to deposit formation when exposed to high temperatures, air and catalytic metals. This screening method recreates comparable conditions to those in the field and makes it possible to improve the longer-term deposit-forming tendency of the next generation of turbine oils.
Technology leadership, including the use of modelling and lubrication science, plays an increasingly central role in product development at Shell. We believe this is key if the next generation of turbine lubricants is to offer further performance enhancements, efficiencies and optimized equipment availability. The way lubrication science responds to end-user experience – as in the case of varnishing – is also something the industry needs to consider; it has an important role to play in ensuring future specification standards guide product innovation and deliver value to customers.
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